12.4 The regular grammar

In order to work properly, the regular grammar used with an Lalr(1) grammar should follow some conventions:

• If a semantic value is to be associated with the token just parsed, the regular grammar should return a pair whose car is the token name (a symbol) and the cdr is the semantic value.
• If there is no value associated with the token, the regular grammar can return just the token name. When used in conjunction with an Lalr grammar, regular grammar should never return #f as a token value. This is specially true when the regular grammar detects the end of parsing. In that case, the regular grammar must not return the #f value. A good way to handle end-of-file is illustrated in the following example:

  (let ((g (regular-grammar ()
               ...
               (else 
                (let ((c (the-failure)))
                   (if (eof-object? c)
                       c
                       (error 'rgc "Illegal character" c))))))
        (l (lalr-grammar ...)))
     (read/lalrp l g (current-input-port)))
  

  This way, the Lalr grammar will automatically handles the end-of-file.

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